What Are the Causes of Red Tides?

The term "red tide" refers to the red sea-water color occasionally seen in coastal waters. Red tides may contain toxins or enough biomass to disturb marine ecosystems; however, some red tides are harmless. Although human activity may increase their frequency or duration, red tides also occur naturally.

Phytoplankton

Sea life depends on growth of plankton, the masses of simple tiny organisms which float in the ocean surface waters. Zooplankton, one of the two components of plankton, consume phytoplankton, the other component. Phytoplankton derive energy from sunlight through photosynthesis. Phytoplankton, including algae, and zooplankton form the foundation of the sea food chain.

Red Tide

Some phytoplankton contain red pigment needed to capture sunlight for photosynthesis. Growth of large numbers of these phytoplankton can occur in algal blooms, rapid increases in phytoplankton population. The pigments result in red sea water coloration, or "red tide."

Algal Blooms

Conditions leading to algal blooms including red tides occur when sea water contains an influx of nutrients, particularly nitrogen and phosphorous, often from river water entering the sea. Human activity may increase sea nitrogen and phosphorous levels. Some red tides contain toxins which directly affect sea life and humans or may be passed up the food chain for example through shellfish. Other red tides may be harmless.

Harmful Algal Blooms

Red tides and other algal blooms also cause environmental damage when masses of their waste and dead plankton sink and provide nutrients for oxygen using bacteria on the sea floor. Oxygen depletion occurs killing higher sea life forms such as fish. Scientists prefer to group toxin-producing or oxygen-depleting red-tide events with other damaging algal blooms, calling all such events "harmful algal blooms," or HABs. HABs may or may not contain red pigment.